Line circuits for key telephone systems are well-known based on the Bell System 400B line card (U.S. Pat. No. 3,239,610 issued Mar. 8, 1966 to Morse et al.) and the 400D card (U.S. Pat. No. 3,436,488 issued Apr. 1, 1969 to Barbato et al.).
Later reference patents such as U.S. Pat. No. 3,647,983 issued Mar. 7, 1972 to Fitzsimons et al. and U.S. Pat. No. 3,715,516 issued Feb. 6, 1973 to Ebrahimi show variations using three relays to produce the same results as the cited Barbato patent, using the same input conditions and producing the same outputs.
Many subsequent patents have issued for key system line circuit patents, the circuits being essentially plug compatible with those noted, thus all the circuits receive and respond to the same input conditions to produce the same output conditions.
Of these, the more recent employ solid state devices to a larger degree with emphasis on one or more optical isolators or optical couplers performing the function of the line relay. Some of these patents include: U.S. Pat. Nos. 3,766,325 issued Oct. 16, 1973 to Hatfield and 3,764,752 issued Oct. 9, 1973 to Yachabach.